1 use super::{Parser, PResult, TokenType};
3 use crate::{maybe_whole, ThinVec};
4 use crate::ast::{self, QSelf, Path, PathSegment, Ident, ParenthesizedArgs, AngleBracketedArgs};
5 use crate::ast::{AnonConst, GenericArg, AssocTyConstraint, AssocTyConstraintKind, BlockCheckMode};
6 use crate::parse::token::{self, Token};
7 use crate::source_map::{Span, BytePos};
12 use errors::{Applicability, pluralise};
14 /// Specifies how to parse a path.
15 #[derive(Copy, Clone, PartialEq)]
17 /// In some contexts, notably in expressions, paths with generic arguments are ambiguous
18 /// with something else. For example, in expressions `segment < ....` can be interpreted
19 /// as a comparison and `segment ( ....` can be interpreted as a function call.
20 /// In all such contexts the non-path interpretation is preferred by default for practical
21 /// reasons, but the path interpretation can be forced by the disambiguator `::`, e.g.
22 /// `x<y>` - comparisons, `x::<y>` - unambiguously a path.
24 /// In other contexts, notably in types, no ambiguity exists and paths can be written
25 /// without the disambiguator, e.g., `x<y>` - unambiguously a path.
26 /// Paths with disambiguators are still accepted, `x::<Y>` - unambiguously a path too.
28 /// A path with generic arguments disallowed, e.g., `foo::bar::Baz`, used in imports,
29 /// visibilities or attributes.
30 /// Technically, this variant is unnecessary and e.g., `Expr` can be used instead
31 /// (paths in "mod" contexts have to be checked later for absence of generic arguments
32 /// anyway, due to macros), but it is used to avoid weird suggestions about expected
33 /// tokens when something goes wrong.
38 /// Parses a qualified path.
39 /// Assumes that the leading `<` has been parsed already.
41 /// `qualified_path = <type [as trait_ref]>::path`
46 /// `<T as U>::F::a<S>` (without disambiguator)
47 /// `<T as U>::F::a::<S>` (with disambiguator)
48 pub(super) fn parse_qpath(&mut self, style: PathStyle) -> PResult<'a, (QSelf, Path)> {
49 let lo = self.prev_span;
50 let ty = self.parse_ty()?;
52 // `path` will contain the prefix of the path up to the `>`,
53 // if any (e.g., `U` in the `<T as U>::*` examples
54 // above). `path_span` has the span of that path, or an empty
55 // span in the case of something like `<T>::Bar`.
56 let (mut path, path_span);
57 if self.eat_keyword(kw::As) {
58 let path_lo = self.token.span;
59 path = self.parse_path(PathStyle::Type)?;
60 path_span = path_lo.to(self.prev_span);
62 path_span = self.token.span.to(self.token.span);
63 path = ast::Path { segments: Vec::new(), span: path_span };
66 // See doc comment for `unmatched_angle_bracket_count`.
67 self.expect(&token::Gt)?;
68 if self.unmatched_angle_bracket_count > 0 {
69 self.unmatched_angle_bracket_count -= 1;
70 debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
73 self.expect(&token::ModSep)?;
75 let qself = QSelf { ty, path_span, position: path.segments.len() };
76 self.parse_path_segments(&mut path.segments, style)?;
78 Ok((qself, Path { segments: path.segments, span: lo.to(self.prev_span) }))
81 /// Parses simple paths.
83 /// `path = [::] segment+`
84 /// `segment = ident | ident[::]<args> | ident[::](args) [-> type]`
87 /// `a::b::C<D>` (without disambiguator)
88 /// `a::b::C::<D>` (with disambiguator)
89 /// `Fn(Args)` (without disambiguator)
90 /// `Fn::(Args)` (with disambiguator)
91 pub fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
92 maybe_whole!(self, NtPath, |path| {
93 if style == PathStyle::Mod &&
94 path.segments.iter().any(|segment| segment.args.is_some()) {
95 self.diagnostic().span_err(path.span, "unexpected generic arguments in path");
100 let lo = self.meta_var_span.unwrap_or(self.token.span);
101 let mut segments = Vec::new();
102 let mod_sep_ctxt = self.token.span.ctxt();
103 if self.eat(&token::ModSep) {
104 segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
106 self.parse_path_segments(&mut segments, style)?;
108 Ok(Path { segments, span: lo.to(self.prev_span) })
111 /// Like `parse_path`, but also supports parsing `Word` meta items into paths for
112 /// backwards-compatibility. This is used when parsing derive macro paths in `#[derive]`
114 pub fn parse_path_allowing_meta(&mut self, style: PathStyle) -> PResult<'a, Path> {
115 let meta_ident = match self.token.kind {
116 token::Interpolated(ref nt) => match **nt {
117 token::NtMeta(ref item) => match item.tokens.is_empty() {
118 true => Some(item.path.clone()),
125 if let Some(path) = meta_ident {
129 self.parse_path(style)
132 crate fn parse_path_segments(
134 segments: &mut Vec<PathSegment>,
136 ) -> PResult<'a, ()> {
138 let segment = self.parse_path_segment(style)?;
139 if style == PathStyle::Expr {
140 // In order to check for trailing angle brackets, we must have finished
141 // recursing (`parse_path_segment` can indirectly call this function),
142 // that is, the next token must be the highlighted part of the below example:
144 // `Foo::<Bar as Baz<T>>::Qux`
147 // As opposed to the below highlight (if we had only finished the first
150 // `Foo::<Bar as Baz<T>>::Qux`
153 // `PathStyle::Expr` is only provided at the root invocation and never in
154 // `parse_path_segment` to recurse and therefore can be checked to maintain
156 self.check_trailing_angle_brackets(&segment, token::ModSep);
158 segments.push(segment);
160 if self.is_import_coupler() || !self.eat(&token::ModSep) {
166 pub(super) fn parse_path_segment(&mut self, style: PathStyle) -> PResult<'a, PathSegment> {
167 let ident = self.parse_path_segment_ident()?;
169 let is_args_start = |token: &Token| match token.kind {
170 token::Lt | token::BinOp(token::Shl) | token::OpenDelim(token::Paren)
171 | token::LArrow => true,
174 let check_args_start = |this: &mut Self| {
175 this.expected_tokens.extend_from_slice(
176 &[TokenType::Token(token::Lt), TokenType::Token(token::OpenDelim(token::Paren))]
178 is_args_start(&this.token)
181 Ok(if style == PathStyle::Type && check_args_start(self) ||
182 style != PathStyle::Mod && self.check(&token::ModSep)
183 && self.look_ahead(1, |t| is_args_start(t)) {
184 // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
185 // it isn't, then we reset the unmatched angle bracket count as we're about to start
186 // parsing a new path.
187 if style == PathStyle::Expr {
188 self.unmatched_angle_bracket_count = 0;
189 self.max_angle_bracket_count = 0;
192 // Generic arguments are found - `<`, `(`, `::<` or `::(`.
193 self.eat(&token::ModSep);
194 let lo = self.token.span;
195 let args = if self.eat_lt() {
197 let (args, constraints) =
198 self.parse_generic_args_with_leaning_angle_bracket_recovery(style, lo)?;
200 let span = lo.to(self.prev_span);
201 AngleBracketedArgs { args, constraints, span }.into()
204 let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
205 let span = ident.span.to(self.prev_span);
206 let output = if self.eat(&token::RArrow) {
207 Some(self.parse_ty_common(false, false, false)?)
211 ParenthesizedArgs { inputs, output, span }.into()
214 PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
216 // Generic arguments are not found.
217 PathSegment::from_ident(ident)
221 pub(super) fn parse_path_segment_ident(&mut self) -> PResult<'a, Ident> {
222 match self.token.kind {
223 token::Ident(name, _) if name.is_path_segment_keyword() => {
224 let span = self.token.span;
226 Ok(Ident::new(name, span))
228 _ => self.parse_ident(),
232 /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
233 /// For the purposes of understanding the parsing logic of generic arguments, this function
234 /// can be thought of being the same as just calling `self.parse_generic_args()` if the source
235 /// had the correct amount of leading angle brackets.
237 /// ```ignore (diagnostics)
238 /// bar::<<<<T as Foo>::Output>();
239 /// ^^ help: remove extra angle brackets
241 fn parse_generic_args_with_leaning_angle_bracket_recovery(
245 ) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
246 // We need to detect whether there are extra leading left angle brackets and produce an
247 // appropriate error and suggestion. This cannot be implemented by looking ahead at
248 // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
249 // then there won't be matching `>` tokens to find.
251 // To explain how this detection works, consider the following example:
253 // ```ignore (diagnostics)
254 // bar::<<<<T as Foo>::Output>();
255 // ^^ help: remove extra angle brackets
258 // Parsing of the left angle brackets starts in this function. We start by parsing the
259 // `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
262 // *Upcoming tokens:* `<<<<T as Foo>::Output>;`
263 // *Unmatched count:* 1
264 // *`parse_path_segment` calls deep:* 0
266 // This has the effect of recursing as this function is called if a `<` character
267 // is found within the expected generic arguments:
269 // *Upcoming tokens:* `<<<T as Foo>::Output>;`
270 // *Unmatched count:* 2
271 // *`parse_path_segment` calls deep:* 1
273 // Eventually we will have recursed until having consumed all of the `<` tokens and
274 // this will be reflected in the count:
276 // *Upcoming tokens:* `T as Foo>::Output>;`
277 // *Unmatched count:* 4
278 // `parse_path_segment` calls deep:* 3
280 // The parser will continue until reaching the first `>` - this will decrement the
281 // unmatched angle bracket count and return to the parent invocation of this function
282 // having succeeded in parsing:
284 // *Upcoming tokens:* `::Output>;`
285 // *Unmatched count:* 3
286 // *`parse_path_segment` calls deep:* 2
288 // This will continue until the next `>` character which will also return successfully
289 // to the parent invocation of this function and decrement the count:
291 // *Upcoming tokens:* `;`
292 // *Unmatched count:* 2
293 // *`parse_path_segment` calls deep:* 1
295 // At this point, this function will expect to find another matching `>` character but
296 // won't be able to and will return an error. This will continue all the way up the
297 // call stack until the first invocation:
299 // *Upcoming tokens:* `;`
300 // *Unmatched count:* 2
301 // *`parse_path_segment` calls deep:* 0
303 // In doing this, we have managed to work out how many unmatched leading left angle
304 // brackets there are, but we cannot recover as the unmatched angle brackets have
305 // already been consumed. To remedy this, we keep a snapshot of the parser state
306 // before we do the above. We can then inspect whether we ended up with a parsing error
307 // and unmatched left angle brackets and if so, restore the parser state before we
308 // consumed any `<` characters to emit an error and consume the erroneous tokens to
309 // recover by attempting to parse again.
311 // In practice, the recursion of this function is indirect and there will be other
312 // locations that consume some `<` characters - as long as we update the count when
313 // this happens, it isn't an issue.
315 let is_first_invocation = style == PathStyle::Expr;
316 // Take a snapshot before attempting to parse - we can restore this later.
317 let snapshot = if is_first_invocation {
323 debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
324 match self.parse_generic_args() {
325 Ok(value) => Ok(value),
326 Err(ref mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
327 // Cancel error from being unable to find `>`. We know the error
328 // must have been this due to a non-zero unmatched angle bracket
332 // Swap `self` with our backup of the parser state before attempting to parse
333 // generic arguments.
334 let snapshot = mem::replace(self, snapshot.unwrap());
337 "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
338 snapshot.count={:?}",
339 snapshot.unmatched_angle_bracket_count,
342 // Eat the unmatched angle brackets.
343 for _ in 0..snapshot.unmatched_angle_bracket_count {
347 // Make a span over ${unmatched angle bracket count} characters.
348 let span = lo.with_hi(
349 lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count)
355 "unmatched angle bracket{}",
356 pluralise!(snapshot.unmatched_angle_bracket_count)
362 "remove extra angle bracket{}",
363 pluralise!(snapshot.unmatched_angle_bracket_count)
366 Applicability::MachineApplicable,
370 // Try again without unmatched angle bracket characters.
371 self.parse_generic_args()
377 /// Parses (possibly empty) list of lifetime and type arguments and associated type bindings,
378 /// possibly including trailing comma.
379 fn parse_generic_args(&mut self) -> PResult<'a, (Vec<GenericArg>, Vec<AssocTyConstraint>)> {
380 let mut args = Vec::new();
381 let mut constraints = Vec::new();
382 let mut misplaced_assoc_ty_constraints: Vec<Span> = Vec::new();
383 let mut assoc_ty_constraints: Vec<Span> = Vec::new();
385 let args_lo = self.token.span;
388 if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
389 // Parse lifetime argument.
390 args.push(GenericArg::Lifetime(self.expect_lifetime()));
391 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
392 } else if self.check_ident() && self.look_ahead(1,
393 |t| t == &token::Eq || t == &token::Colon) {
394 // Parse associated type constraint.
395 let lo = self.token.span;
396 let ident = self.parse_ident()?;
397 let kind = if self.eat(&token::Eq) {
398 AssocTyConstraintKind::Equality {
399 ty: self.parse_ty()?,
401 } else if self.eat(&token::Colon) {
402 AssocTyConstraintKind::Bound {
403 bounds: self.parse_generic_bounds(Some(self.prev_span))?,
408 let span = lo.to(self.prev_span);
409 constraints.push(AssocTyConstraint {
410 id: ast::DUMMY_NODE_ID,
415 assoc_ty_constraints.push(span);
416 } else if self.check_const_arg() {
417 // Parse const argument.
418 let expr = if let token::OpenDelim(token::Brace) = self.token.kind {
419 self.parse_block_expr(
420 None, self.token.span, BlockCheckMode::Default, ThinVec::new()
422 } else if self.token.is_ident() {
423 // FIXME(const_generics): to distinguish between idents for types and consts,
424 // we should introduce a GenericArg::Ident in the AST and distinguish when
425 // lowering to the HIR. For now, idents for const args are not permitted.
426 if self.token.is_bool_lit() {
427 self.parse_literal_maybe_minus()?
430 self.fatal("identifiers may currently not be used for const generics")
434 self.parse_literal_maybe_minus()?
436 let value = AnonConst {
437 id: ast::DUMMY_NODE_ID,
440 args.push(GenericArg::Const(value));
441 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
442 } else if self.check_type() {
443 // Parse type argument.
444 args.push(GenericArg::Type(self.parse_ty()?));
445 misplaced_assoc_ty_constraints.append(&mut assoc_ty_constraints);
450 if !self.eat(&token::Comma) {
455 // FIXME: we would like to report this in ast_validation instead, but we currently do not
456 // preserve ordering of generic parameters with respect to associated type binding, so we
457 // lose that information after parsing.
458 if misplaced_assoc_ty_constraints.len() > 0 {
459 let mut err = self.struct_span_err(
460 args_lo.to(self.prev_span),
461 "associated type bindings must be declared after generic parameters",
463 for span in misplaced_assoc_ty_constraints {
466 "this associated type binding should be moved after the generic parameters",
472 Ok((args, constraints))